The widespread popularity of the Internet in recent years has been closely linked with the development of wireless communication technology and with the popularity of mobile telephones and mobile communication terminals in vehicles and the like that have a wireless communication function that enables them to connect to the Internet. Accordingly, services for sharing information contents (a generic term indicating images, music, video, text, messages and the like) that are based on an ad hoc network which forms a network without any communication infrastructure being required between multiple mobile communication terminals are showing genuine promise, and their usefulness as information transmission devices in cities or in disaster areas is anticipated.
However, because this type of ad hoc network is based on wireless communication, unlike the Internet which is formed by an interconnected wire network, the communication performance (i.e., the communication zone) is considerably limited by the environment surrounding the mobile communication terminals, and by obstacles and radio wave interference and the like. In addition, the communication topology of a formed ad hoc network is changed if the mobile communication terminals move, and route disconnections and the like occur frequently. As a result, it is not possible to hope for stable communication such as that obtainable from a wire network.
In information sharing that takes place when the communication performance (i.e., the communication zone) is limited, and in a communication environment where the communication path cannot be guaranteed, it is effective for information to be exchanged beforehand at a time when communication with other mobile communication terminals is possible. One method that has been proposed to achieve this is a method in which, when the movements of two mobile communication terminals cause them to come into mutual contact, they exchange information with each other, and synchronize the information they are holding (see, for example, Non-patent document 1).
In Non-patent document 1, the “Epidemic” method for synchronizing held information between all mobile communication terminals is proposed. The Epidemic method is characterized in that information is synchronized based on summary vectors (referred to below as ‘SV’) which are messages that summarize the information held by a terminal and notify other mobile communication terminals about this information.
In the Epidemic method, when a mobile communication terminal within a network detects another mobile communication terminal, they exchange summary vectors with each other, and ascertain the contents of the SV of the other mobile communication terminal. Based on the received SV, each mobile communication terminal then examines any contents that it does not itself hold, and attempts to acquire those contents. For example, as is shown in FIG. 17, a terminal T1 and a terminal T2 that have come into mutual contact as a result of moving their positions exchange SV with each other, and the terminal T2 verifies any differential contents by comparing the SV of the terminal T1 with its own SV and determining whether or not the terminal T1 has any contents that it does not itself hold. At this time, because the terminal T2 does not hold the contents B, it acquires the contents B by transmitting an acquisition request for the contents B to the terminal T1. The information held by all the terminals is synchronized as a result of all of the terminals within a network repeating this operation.
As has been described above, the Epidemic method is a method in which a comparison of the contents held by each terminal is made by means of the SV so that each terminal confirms whether there are any contents it does not itself hold, and then exchanges contents. These SV can also be used for replacing information within the network (i.e., terminal position coordinates and held contents information) with the information shown by the SV, and for verifying information existing on the network, and for transmitting requests to terminals within the network.